1. Technical Field of the Invention
The present invention relates generally to spark plugs for internal combustion engines. More particularly, the invention relates to an improved structure of a spark plug for an internal combustion engine of an automotive vehicle which ensures high performance and a long service life of the spark plug.
2. Description of the Related Art
Conventional spark plugs for use in internal combustion engines generally include a metal shell, an insulator, a center electrode, and a ground electrode.
The metal shell has a threaded portion for fitting the spark plug into a combustion chamber of the engine. The insulator has a center bore formed therein, and is fixed in the metal shell such that an end thereof protrudes from an end of the metal shell. The center electrode is secured in the center bore of the insulator such that an end thereof protrudes from the end of the insulator. The ground electrode has a side surface, and is joined to the end of the metal shell such that the side surface thereof is opposed to and spaced from the end of the center electrode so as to form a spark gap therebetween.
In recent years, an increase of compression ratios of internal combustion engines has been pursued for the purpose of increasing power output and improving fuel economies. However, at the same time, such an increase of compression ratio causes an increase of required spark voltage (i.e., the electric voltage required for sparking) of a spark plug.
The increased required spark voltage for the spark plug implies that it becomes difficult to generate sparks in the spark gap of the spark plug. Thus, instead of normal sparks being generated in the spark gap, “side sparks” can be generated.
The side spark is a spark which creeps from the center electrode of a spark plug along an outer surface of the insulator, and flies to the metal shell of the spark plug. More specifically, the side spark flies over the gap between the outer surface of the insulator and an inner surface of the metal shell, thus resulting in a misfire of the spark plug. Accordingly, when the side spark is generated, the performance of the engine employing the spark plug will drop.
On the other hand, in order to increase the power output of an internal combustion engine, it is generally required to increase the sizes of valves used in connection with the intake manifolds and exhaust manifolds for the engine and to secure a water jacket for the cooling of the engine. Consequently, the space available for installing a spark plug to the engine is decreased, and accordingly, it is desired to minimize the size of the spark plug.
The minimization of the spark plug results in a decreased size of an air pocket, which is the space between an outer surface of the insulator and an inner surface of the metal shell at the end of the metal shell to which the ground electrode is joined. The decreased size of air pocket can generate side sparks in the spark plug, in addition to an increase of required spark voltage for the spark plug as described above.
Therefore, it is required to keep the size of the air pocket in a spark plug above a certain level so as to prevent generation of side sparks. However, on the other hand, when the radial thickness of the insulator of the spark plug is sacrificed for keeping the size of the air pocket in minimization of the spark plug, the withstand voltage of the spark plug will be decreased; the decreased withstand voltage can cause a dielectric breakdown of the spark plug.
Accordingly, when minimizing a spark plug, there is a trade-off between preventing generation of side sparks in the spark plug and securing withstand voltage of the spark plug.
As a solution to such a trade-off, a spark plug is proposed in Japanese Unexamined Patent Publication No. 2000-243535, which has a structure with appropriately specified parameters such as the radial thickness of an insulator and the air pocket size in the spark plug as described above.
In addition to pursuing the high performance of internal combustion engines as described above, a long service life for those engines has also been pursued. For example, it was required to secure an actual mileage of about 100,000 km for an engine in the past; now, however, 200,000 km is required.
Under such circumstances, the inventors of the present invention have investigated the spark plug proposed in Japanese Unexamined Patent Publication No. 2000-243535. As a result, the inventors have found that when the spark plug is used over a long period of time, it is not possible to reliably eliminate side sparks in the spark plug.
Specifically, when the spark plug is used for a long period of time above 200,000 km, the center and ground electrodes of the spark plug will be considerably worn down, so that the spark gap therebetween is largely increased. Then, the required spark voltage of the spark plug is also increased due to the increased spark gap, thus facilitating generation of side sparks in the spark plug. Consequently, the structure of the spark plug is unable to secure a high performance and a long service life for the spark plug.